Rhizomania, "root madness" is a severe disease of sugar beet caused by beet necrotic yellow vein virus (BNYVV) and transmitted by the soilborne fungus Polymyxa betae. The disease is widely distributed in many countries and is economically devastating to a sugar beet crop, causing severe loss in root yield and sugar content of the infected plants. In naturally infected plants, BNYVV is normally confined to the roots, and causes massive proliferation of the lateral rootlets of taproots as well as other abnormalities of the root system.
Novel isolates of beet soilborne mosaic virus, BSBMV (previously called Texas 7), have now been identified as non-virulent or mildly virulent viruses closely related to BNYVV. When co-inoculated with BNYVV in a susceptible host, BSBMV dominates over BNYVV, and interferes with infection by the more pathogenic BNYVV. A method for preferentially incorporating BSBMV into plants and thereby excluding or inhibiting infection and disease caused by the more virulent BNYVV would be extremely useful, especially in the sugar beet industry. Such a method would also be of great utility for inoculation of plants against a variety of severe diseases caused by furoviruses.
Similarly, a method for easily incorporating a competing, non-virulent or mildly virulent virus into a plant to confer protection against a more virulent pathogen would be highly desirable. Such a method should be easily applied in the field, permit specific application to a plant or field of choice, and be stable over time. The methods of the present invention provide such easily administered and controlled application of a beneficial virus or other desired nucleic acid sequence to a host plant.